 Most significant is the stiff, grey-blue London Clay, a marine deposit which is well known for the fossils it contains and can be over 150 metres thick beneath the city.This supports most of the deep foundations and tunnels that exist under London.Also in this area are the Lambeth Group (formerly known as the Woolwich and Reading Beds, though those beds have been recently deprecated in 1994 in favour of the Lambeth Group by the British Geological Survey to conform with new standards and to allow scope for more detailed subdivisions) and Thanet Sands.